1. Field of the Invention
The present invention relates to ECL circuits in general and to an ECL circuit comprising an auxiliary biasing circuit for emphasizing the "turn-off" bias of a deselected ECL output transistor coupled to a data bus in parallel with other deselected ECL output transistors in other ECL circuits in particular.
2. Description of Prior Art
In conventional ECL circuits comprising a plurality of N output transistors coupled in parallel to a data bus terminated by a load corresponding to the characteristic impedance of the bus, it has been the practice to selectively deselect or "turn-off" N-1 of the output transistors when writing data to the bus using the remaining or selected output transistor. The remaining, or selected, output transistor is, of course, turned on and off as a function of the data being written to the bus.
In a typical ECL circuit, as described above, comprising an NPN output transistor, turning off the output transistor involves driving the base of the transistor from 0 volts to -0.8 volts. With -0.8 volts on the base and a -0.8 volt V.sub.be drop across the transistor, the emitter potential, i.e. the potential applied to the load, is switched from -0.8 volts to -1.6 volts. The other end of the load is usually at a fixed D.C. voltage, e.g. -2.0 volts.
Ordinarily, a -1.6 volt potential on the emitter of an ECL output transistor and consequently a +0.4 volt drop, i.e. [-1.6-(-2)], across the load is sufficient to turn off a following ECL transistor being driven thereby as well as provide an adequate level of immunity to noise on the bus. However, when a plurality of ECL output transistors are coupled in parallel to the same bus, it has been found that when all of them are "turned off", each of them still conducts enough current to significantly raise the potential drop across the load and thereby reduce the immunity to noise of the bus to inadequate levels. For example, with four ECL output transistors coupled in parallel to a data bus, the potential drop on the bus at a high temperature, e.g. 150.degree. C., may be raised by as much as 50 millivolts from -1.6 volts to -1.55 volts thus reducing the noise immunity of the bus by as much as 50 millivolts.